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Posted: Jul 17th, 2007
Nanobionics - where the boundaries between electronics and biology become fuzzy
(Nanowerk Spotlight) In case you are not old enough to remember the TV series The Six Million Dollar Man during the 1970s, the show was about an astronaut, Steve Austin, who got severely injured during a crash and became a guinea pig for bionics experiments by the CIA. In an operation that cost six million dollars, his right arm, both legs and the left eye are replaced by bionic (cybernetic) implants that vastly enhanced his strength, speed and vision. Never mind Hollywood, though, but bionics - a word formed from biology and electronics - has become a serious research field. In particular the development of artificial muscles is progressing rapidly. Nature’s solution to producing fast contracting muscles is to use nanotechnology. The challenge for scientists is to mimic the intricacy of natural muscle in their artificial-muscle systems. As material scientists and engineers delve into the nanodomain, the boundaries between electronics and biology become fuzzy and this is exactly what they want: a seamless transition between the hard world of electronics and the soft world of biology.
Cochlear implants – electrodes implanted in the cochlea to stimulate auditory nerves – are the most successful bionic prosthetics to date. Advanced bionic components may be within our grasp, but putting them all together in a real-life super powered Steve Austin will remain the stuff of sci-fi for many years to come. While a bionic person may be theoretically feasible, the major problem lies in overcoming tissue-material interfaces. Not only would scientists have to build artificial muscles and organs that resemble the real thing in power, function and size, they also would need to be able to 'connect' these artificial devices to the body's neural network.
"Effectively bridging the interface between electronics and biology is critically dependent on advances in new materials" write Dr. Gordon Wallace and Dr. Geoffrey Spinks in their recent paper "Conducting polymers – bridging the bionic interface" in Soft Matter. "The discovery of inherently conducting polymers (ICPs) in the late 1970s revolutionized how we think about electronic conductors. Now at our disposal are electronic conductors that are organic in nature, and also a significant portion of the conducting polymer (the dopant) can be a biological entity. The soft character of ICPs coupled with their dynamic nature provides an extra dimension in designing interfaces between the hard, digital electronics world and the soft, amorphous world of biological systems. The use of ICPs has the potential to impact all levels of bionics."
Basically, there are three levels of biocommunications where electronics and biology could interface: molecular, cellular and skeletal. For any implanted bionic material it is the initial interactions at the biomolecular level that will determine longer term performance. While bionics is often associated with skeletal level enhancements, electronic communication with living cells is of interest with a view to improving the performance of implants for tissue engineering or bone regrowth. It is also critical to the performance of implants such as the bionic ears or eyes.
The development of artificial muscles is one of the key areas for bionic enhancements or replacements. The quest to build artificial muscles
using ICPs can be traced to Ray H. Baughman's paper in 1990 "Conjugated polymeric materials: opportunities in electronics, optoelectronics and molecular electronics,".
The ACES researchers say that, when the performance of natural muscle is compared with conventional mechanical drive systems (motors, pneumatics, hydraulics), the sophistication of muscle as a machine can be appreciated.
Interestingly, the force generated from a wide variety of actuator materials and devices has been found to be surprisingly invariant when compared with the actuator mass. A few years back, a comparison of the force-to-weight ratio of various organisms and machines found a striking similarity, with the force scaling linearly with mass over 20 orders of magnitude – from individual protein molecules to rocket engines ("Molecules, muscles, and machines: Universal performance characteristics of motors"). Remarkably, this finding indicates that most of the motors used by humans and animals for transportation have a common upper limit of mass-specific net force output that is independent of materials and mechanisms. Therefore any actuating device produces the
same force per mass regardless of the material from which it is constructed and the mechanism by which it operates.
This study also makes clear that biological systems dominate at the small mass, small force, range. In contrast, human-made machines dominate at the large mass range.
Wallace and Spinks point out that human developed systems are typically built for high force, high speed operation needed in transport (cars, rockets, etc) and heavy lifting equipment (cranes, elevators etc). "At this scale, nature loses out to human technology: planes fly faster than birds; and forklifts are stronger than elephants. However, very few human-made machines are found at less than 0.1 kg in mass. The reason for the lack of small sized engines and motors is no doubt partly related to their complexity, making fabrication costs very high for micro-machines."
Research continues in the area of micro-fabrication of motors and engines, with the MIT gas turbine engine a prominent example; the 2 g engine produces 0.2 N thrust force and a power output of ∼10 W.
"Another reason, however, is that the efficiency of electric motors decreases as the size of the motor gets smaller" say the authors. "Thus, efficient small-scale motors require different operating mechanisms, such as the muscle systems ubiquitous in nature."
Even with all the improvement in artificial-muscle performance that has occurred in recent years, there is still some way to go before they can match natural muscle in terms of speed, efficiency and control.
Nature's solution to producing fast contracting muscles is to use nanotechnology. The challenge for scientists is to mimic the intricacy of natural muscle in artificial-muscle systems: "The structure of skeletal muscle is hierarchical, with whole muscles consisting of many parallel muscle fibres (each an individual cell): each muscle fibre made of many parallel myofibrils; and each myofibril consisting of many parallel myofilaments
containing different proteins. It is the action of the proteins that is primarily responsible for muscle contraction, although the careful nano to macro structure of muscles must also contribute to the advanced performance."
Fabricating nanostructured systems with a high surface-area to volume ratio (like bundled nanofibres) that allow a high speed response without compromising the large force and movement is therefore one of the challenges facing artificial-muscle engineers. It is the nanostructured interfaces from conducting polymer nanocomponents that will make more effective biomolecular and biocellular interfaces for bionic applications possible.